Method and apparatus for handling radio resources for device-to-device operation in wireless communication system

ABSTRACT

A method and apparatus for handling radio resources upon mobility in a wireless communication system is provided. In one embodiment, a user equipment (UE) receives information on radio resources allowed for a device-to-device (D2D) operation in a target cell during a handover procedure, and perform the D2D operation in the target cell based on received information on radio resources. In another embodiment, a UE performs a handover procedure or a connection re-establishment procedure, suspends using D2D resources, and resumes suspended D2D resources.

TECHNICAL FIELD

The present invention relates to wireless communications, and morespecifically, to a method and apparatus for handling radio resources fora device-to-device (D2D) operation upon mobility in a wirelesscommunication system.

BACKGROUND ART

Universal mobile telecommunications system (UMTS) is a 3^(rd) generation(3G) asynchronous mobile communication system operating in wideband codedivision multiple access (WCDMA) based on European systems, globalsystem for mobile communications (GSM) and general packet radio services(GPRS). A long-term evolution (LTE) of UMTS is under discussion by the3^(rd) generation partnership project (3GPP) that standardized UMTS.

The 3GPP LTE is a technology for enabling high-speed packetcommunications. Many schemes have been proposed for the LTE objectiveincluding those that aim to reduce user and provider costs, improveservice quality, and expand and improve coverage and system capacity.The 3GPP LTE requires reduced cost per bit, increased serviceavailability, flexible use of a frequency band, a simple structure, anopen interface, and adequate power consumption of a terminal as anupper-level requirement.

Recently, there has been a surge of interest in supporting directdevice-to-device (D2D) communication. This new interest is motivated byseveral factors, including the popularity of proximity-based services,driven largely by social networking applications, and the crushing datademands on cellular spectrum, much of which is localized traffic, andthe under-utilization of uplink frequency bands. 3GPP is targeting theavailability of D2D communication in LTE rel-12 to enable LTE become acompetitive broadband communication technology for public safetynetworks, used by first responders. Due to the legacy issues and budgetconstraints, current public safety networks are still mainly based onobsolete 2G technologies while commercial networks are rapidly migratingto LTE. This evolution gap and the desire for enhanced services have ledto global attempts to upgrade existing public safety networks. Comparedto commercial networks, public safety networks have much more stringentservice requirements (e.g., reliability and security) and also requiredirect communication, especially when cellular coverage fails or is notavailable. This essential direct mode feature is currently missing inLTE.

From a technical perspective, exploiting the nature proximity ofcommunicating devices may provide multiple performance benefits. First,D2D user equipments (UEs) may enjoy high data rate and low end-to-enddelay due to the short-range direct communication. Second, it is moreresource-efficient for proximate UEs to communicate directly with eachother, versus routing through an evolved NodeB (eNB) and possibly thecore network. In particular, compared to normal downlink/uplink cellularcommunication, direct communication saves energy and improves radioresource utilization. Third, switching from an infrastructure path to adirect path offloads cellular traffic, alleviating congestion, and thusbenefitting other non-D2D UEs as well. Other benefits may be envisionedsuch as range extension via UE-to-UE relaying.

While the UE performs a D2D operation with other UEs, handover of the UEmay occur. In this case, information on radio resources used for a D2Doperation should be informed from a source cell to a target cell.Accordingly, a method for handling radio resources used for a D2Doperation upon mobility may be required.

SUMMARY OF INVENTION Technical Problem

The present invention provides a method and apparatus for handling radioresources for a device-to-device (D2D) operation upon mobility in awireless communication system. The present invention provides a methodfor performing a D2D operation in a target cell by using radio resourcesallowed for the D2D operation in the target cell. The present inventionprovides a method for suspending and resuming radio resources for a D2Doperation.

Solution to Problem

In an aspect, a method for performing, by a user equipment (UE), adevice-to-device (D2D) operation in a wireless communication system isprovided. The method includes receiving information on radio resourcesallowed for a D2D operation in a target cell during a handoverprocedure, and performing the D2D operation in the target cell based onreceived information on radio resources.

In another aspect, a user equipment (UE) in a wireless communicationsystem is provided. The UE includes a radio frequency (RF) unit fortransmitting or receiving a radio signal, and a processor coupled to theRF unit, and configured to receive information on radio resourcesallowed for a device-to-device (D2D) operation in a target cell during ahandover procedure, and perform the D2D operation in the target cellbased on received information on radio resources.

In another aspect, a method for performing, by a user equipment (UE),device-to-device (D2D) operation in a wireless communication system isprovided. The method includes performing a handover procedure or aconnection re-establishment procedure, suspending using D2D resources,and resuming suspended D2D resources.

Advantageous Effects of Invention

Radio resources for a D2D operation can be handled effectively uponmobility.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows LTE system architecture.

FIG. 2 shows a block diagram of architecture of a typical E-UTRAN and atypical EPC.

FIG. 3 shows a block diagram of a user plane protocol stack and acontrol plane protocol stack of an LTE system.

FIG. 4 shows an example of a physical channel structure.

FIG. 5 and FIG. 6 show ProSe direct communication scenarios without arelay.

FIG. 7 shows reference architecture for ProSe.

FIG. 8 shows an example of one-step ProSe direct discovery procedure.

FIG. 9 shows an example of two-steps ProSe direct discovery procedure.

FIG. 10 shows an example of a method for performing a D2D operationaccording to an embodiment of the present invention.

FIG. 11 shows an example of a method for performing a D2D operationaccording to another embodiment of the present invention.

FIG. 12 is a block diagram showing wireless communication system toimplement an embodiment of the present invention.

MODE FOR THE INVENTION

The technology described below can be used in various wirelesscommunication systems such as code division multiple access (CDMA),frequency division multiple access (FDMA), time division multiple access(TDMA), orthogonal frequency division multiple access (OFDMA), singlecarrier frequency division multiple access (SC-FDMA), etc. The CDMA canbe implemented with a radio technology such as universal terrestrialradio access (UTRA) or CDMA-2000. The TDMA can be implemented with aradio technology such as global system for mobile communications(GSM)/general packet ratio service (GPRS)/enhanced data rate for GSMevolution (EDGE). The OFDMA can be implemented with a radio technologysuch as institute of electrical and electronics engineers (IEEE) 802.11(Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, evolved UTRA (E-UTRA), etc.IEEE 802.16m is evolved from IEEE 802.16e, and provides backwardcompatibility with a system based on the IEEE 802.16e. The UTRA is apart of a universal mobile telecommunication system (UMTS). 3^(rd)generation partnership project (3GPP) long term evolution (LTE) is apart of an evolved UMTS (E-UMTS) using the E-UTRA. The 3GPP LTE uses theOFDMA in a downlink and uses the SC-FDMA in an uplink LTE-advanced(LTE-A) is an evolution of the LTE.

For clarity, the following description will focus on LTE-A. However,technical features of the present invention are not limited thereto.

FIG. 1 shows LTE system architecture. The communication network iswidely deployed to provide a variety of communication services such asvoice over internet protocol (VoIP) through IMS and packet data.

Referring to FIG. 1, the LTE system architecture includes one or moreuser equipment (UE; 10), an evolved-UMTS terrestrial radio accessnetwork (E-UTRAN) and an evolved packet core (EPC). The UE 10 refers toa communication equipment carried by a user. The UE 10 may be fixed ormobile, and may be referred to as another terminology, such as a mobilestation (MS), a user terminal (UT), a subscriber station (SS), awireless device, etc.

The E-UTRAN includes one or more evolved node-B (eNB) 20, and aplurality of UEs may be located in one cell. The eNB 20 provides an endpoint of a control plane and a user plane to the UE 10. The eNB 20 isgenerally a fixed station that communicates with the UE 10 and may bereferred to as another terminology, such as a base station (BS), a basetransceiver system (BTS), an access point, etc. One eNB 20 may bedeployed per cell. There are one or more cells within the coverage ofthe eNB 20. A single cell is configured to have one of bandwidthsselected from 1.25, 2.5, 5, 10, and 20 MHz, etc., and provides downlinkor uplink transmission services to several UEs. In this case, differentcells can be configured to provide different bandwidths.

Hereinafter, a downlink (DL) denotes communication from the eNB 20 tothe UE 10, and an uplink (UL) denotes communication from the UE 10 tothe eNB 20. In the DL, a transmitter may be a part of the eNB 20, and areceiver may be a part of the UE 10. In the UL, the transmitter may be apart of the UE 10, and the receiver may be a part of the eNB 20.

The EPC includes a mobility management entity (MME) which is in chargeof control plane functions, and a system architecture evolution (SAE)gateway (S-GW) which is in charge of user plane functions. The MME/S-GW30 may be positioned at the end of the network and connected to anexternal network. The MME has UE access information or UE capabilityinformation, and such information may be primarily used in UE mobilitymanagement. The S-GW is a gateway of which an endpoint is an E-UTRAN.The MME/S-GW 30 provides an end point of a session and mobilitymanagement function for the UE 10. The EPC may further include a packetdata network (PDN) gateway (PDN-GW). The PDN-GW is a gateway of which anendpoint is a PDN.

The MME provides various functions including non-access stratum (NAS)signaling to eNBs 20, NAS signaling security, access stratum (AS)security control, Inter core network (CN) node signaling for mobilitybetween 3GPP access networks, idle mode UE reachability (includingcontrol and execution of paging retransmission), tracking area listmanagement (for UE in idle and active mode), P-GW and S-GW selection,MME selection for handovers with MME change, serving GPRS support node(SGSN) selection for handovers to 2G or 3G 3GPP access networks,roaming, authentication, bearer management functions including dedicatedbearer establishment, support for public warning system (PWS) (whichincludes earthquake and tsunami warning system (ETWS) and commercialmobile alert system (CMAS)) message transmission. The S-GW host providesassorted functions including per-user based packet filtering (by e.g.,deep packet inspection), lawful interception, UE Internet protocol (IP)address allocation, transport level packet marking in the DL, UL and DLservice level charging, gating and rate enforcement, DL rate enforcementbased on APN-AMBR. For clarity MME/S-GW 30 will be referred to hereinsimply as a “gateway,” but it is understood that this entity includesboth the MME and S-GW.

Interfaces for transmitting user traffic or control traffic may be used.The UE 10 and the eNB 20 are connected by means of a Uu interface. TheeNBs 20 are interconnected by means of an X2 interface. Neighboring eNBsmay have a meshed network structure that has the X2 interface. The eNBs20 are connected to the EPC by means of an S1 interface. The eNBs 20 areconnected to the MME by means of an S1-MME interface, and are connectedto the S-GW by means of S1-U interface. The S1 interface supports amany-to-many relation between the eNB 20 and the MME/S-GW.

FIG. 2 shows a block diagram of architecture of a typical E-UTRAN and atypical EPC. Referring to FIG. 2, the eNB 20 may perform functions ofselection for gateway 30, routing toward the gateway 30 during a radioresource control (RRC) activation, scheduling and transmitting of pagingmessages, scheduling and transmitting of broadcast channel (BCH)information, dynamic allocation of resources to the UEs 10 in both ULand DL, configuration and provisioning of eNB measurements, radio bearercontrol, radio admission control (RAC), and connection mobility controlin LTE_ACTIVE state. In the EPC, and as noted above, gateway 30 mayperform functions of paging origination, LTE_IDLE state management,ciphering of the user plane, SAE bearer control, and ciphering andintegrity protection of NAS signaling.

FIG. 3 shows a block diagram of a user plane protocol stack and acontrol plane protocol stack of an LTE system. FIG. 3-(a) shows a blockdiagram of a user plane protocol stack of an LTE system, and FIG. 3-(b)shows a block diagram of a control plane protocol stack of an LTEsystem.

Layers of a radio interface protocol between the UE and the E-UTRAN maybe classified into a first layer (L1), a second layer (L2), and a thirdlayer (L3) based on the lower three layers of the open systeminterconnection (OSI) model that is well-known in the communicationsystem. The radio interface protocol between the UE and the E-UTRAN maybe horizontally divided into a physical layer, a data link layer, and anetwork layer, and may be vertically divided into a control plane(C-plane) which is a protocol stack for control signal transmission anda user plane (U-plane) which is a protocol stack for data informationtransmission. The layers of the radio interface protocol exist in pairsat the UE and the E-UTRAN, and are in charge of data transmission of theUu interface.

A physical (PHY) layer belongs to the L1. The PHY layer provides ahigher layer with an information transfer service through a physicalchannel. The PHY layer is connected to a medium access control (MAC)layer, which is a higher layer of the PHY layer, through a transportchannel. A physical channel is mapped to the transport channel. Data istransferred between the MAC layer and the PHY layer through thetransport channel. Between different PHY layers, i.e., a PHY layer of atransmitter and a PHY layer of a receiver, data is transferred throughthe physical channel using radio resources. The physical channel ismodulated using an orthogonal frequency division multiplexing (OFDM)scheme, and utilizes time and frequency as a radio resource.

The PHY layer uses several physical control channels. A physicaldownlink control channel (PDCCH) reports to a UE about resourceallocation of a paging channel (PCH) and a downlink shared channel(DL-SCH), and hybrid automatic repeat request (HARQ) information relatedto the DL-SCH. The PDCCH may carry a UL grant for reporting to the UEabout resource allocation of UL transmission. A physical control formatindicator channel (PCFICH) reports the number of OFDM symbols used forPDCCHs to the UE, and is transmitted in every subframe. A physicalhybrid ARQ indicator channel (PHICH) carries an HARQ acknowledgement(ACK)/non-acknowledgement (NACK) signal in response to UL transmission.A physical uplink control channel (PUCCH) carries UL control informationsuch as HARQ ACK/NACK for DL transmission, scheduling request, and CQI.A physical uplink shared channel (PUSCH) carries a UL-uplink sharedchannel (SCH).

FIG. 4 shows an example of a physical channel structure. A physicalchannel consists of a plurality of subframes in time domain and aplurality of subcarriers in frequency domain. One subframe consists of aplurality of symbols in the time domain. One subframe consists of aplurality of resource blocks (RBs). One RB consists of a plurality ofsymbols and a plurality of subcarriers. In addition, each subframe mayuse specific subcarriers of specific symbols of a corresponding subframefor a PDCCH. For example, a first symbol of the subframe may be used forthe PDCCH. The PDCCH carries dynamic allocated resources, such as aphysical resource block (PRB) and modulation and coding scheme (MCS). Atransmission time interval (TTI) which is a unit time for datatransmission may be equal to a length of one subframe. The length of onesubframe may be 1 ms.

The transport channel is classified into a common transport channel anda dedicated transport channel according to whether the channel is sharedor not. A DL transport channel for transmitting data from the network tothe UE includes a broadcast channel (BCH) for transmitting systeminformation, a paging channel (PCH) for transmitting a paging message, aDL-SCH for transmitting user traffic or control signals, etc. The DL-SCHsupports HARQ, dynamic link adaptation by varying the modulation, codingand transmit power, and both dynamic and semi-static resourceallocation. The DL-SCH also may enable broadcast in the entire cell andthe use of beamforming. The system information carries one or moresystem information blocks. All system information blocks may betransmitted with the same periodicity. Traffic or control signals of amultimedia broadcast/multicast service (MBMS) may be transmitted throughthe DL-SCH or a multicast channel (MCH).

A UL transport channel for transmitting data from the UE to the networkincludes a random access channel (RACH) for transmitting an initialcontrol message, a UL-SCH for transmitting user traffic or controlsignals, etc. The UL-SCH supports HARQ and dynamic link adaptation byvarying the transmit power and potentially modulation and coding. TheUL-SCH also may enable the use of beamforming. The RACH is normally usedfor initial access to a cell.

A MAC layer belongs to the L2. The MAC layer provides services to aradio link control (RLC) layer, which is a higher layer of the MAClayer, via a logical channel. The MAC layer provides a function ofmapping multiple logical channels to multiple transport channels. TheMAC layer also provides a function of logical channel multiplexing bymapping multiple logical channels to a single transport channel. A MACsublayer provides data transfer services on logical channels.

The logical channels are classified into control channels fortransferring control plane information and traffic channels fortransferring user plane information, according to a type of transmittedinformation. That is, a set of logical channel types is defined fordifferent data transfer services offered by the MAC layer. The logicalchannels are located above the transport channel, and are mapped to thetransport channels.

The control channels are used for transfer of control plane informationonly. The control channels provided by the MAC layer include a broadcastcontrol channel (BCCH), a paging control channel (PCCH), a commoncontrol channel (CCCH), a multicast control channel (MCCH) and adedicated control channel (DCCH). The BCCH is a downlink channel forbroadcasting system control information. The PCCH is a downlink channelthat transfers paging information and is used when the network does notknow the location cell of a UE. The CCCH is used by UEs having no RRCconnection with the network. The MCCH is a point-to-multipoint downlinkchannel used for transmitting MBMS control information from the networkto a UE. The DCCH is a point-to-point bi-directional channel used by UEshaving an RRC connection that transmits dedicated control informationbetween a UE and the network.

Traffic channels are used for the transfer of user plane informationonly. The traffic channels provided by the MAC layer include a dedicatedtraffic channel (DTCH) and a multicast traffic channel (MTCH). The DTCHis a point-to-point channel, dedicated to one UE for the transfer ofuser information and can exist in both uplink and downlink. The MTCH isa point-to-multipoint downlink channel for transmitting traffic datafrom the network to the UE.

Uplink connections between logical channels and transport channelsinclude the DCCH that can be mapped to the UL-SCH, the DTCH that can bemapped to the UL-SCH and the CCCH that can be mapped to the UL-SCH.Downlink connections between logical channels and transport channelsinclude the BCCH that can be mapped to the BCH or DL-SCH, the PCCH thatcan be mapped to the PCH, the DCCH that can be mapped to the DL-SCH, andthe DTCH that can be mapped to the DL-SCH, the MCCH that can be mappedto the MCH, and the MTCH that can be mapped to the MCH.

An RLC layer belongs to the L2. The RLC layer provides a function ofadjusting a size of data, so as to be suitable for a lower layer totransmit the data, by concatenating and segmenting the data receivedfrom a higher layer in a radio section. In addition, to ensure a varietyof quality of service (QoS) required by a radio bearer (RB), the RLClayer provides three operation modes, i.e., a transparent mode (TM), anunacknowledged mode (UM), and an acknowledged mode (AM). The AM RLCprovides a retransmission function through an automatic repeat request(ARQ) for reliable data transmission. Meanwhile, a function of the RLClayer may be implemented with a functional block inside the MAC layer.In this case, the RLC layer may not exist.

A packet data convergence protocol (PDCP) layer belongs to the L2. ThePDCP layer provides a function of header compression function thatreduces unnecessary control information such that data being transmittedby employing IP packets, such as IPv4 or IPv6, can be efficientlytransmitted over a radio interface that has a relatively smallbandwidth. The header compression increases transmission efficiency inthe radio section by transmitting only necessary information in a headerof the data. In addition, the PDCP layer provides a function ofsecurity. The function of security includes ciphering which preventsinspection of third parties, and integrity protection which preventsdata manipulation of third parties.

A radio resource control (RRC) layer belongs to the L3. The RLC layer islocated at the lowest portion of the L3, and is only defined in thecontrol plane. The RRC layer takes a role of controlling a radioresource between the UE and the network. For this, the UE and thenetwork exchange an RRC message through the RRC layer. The RRC layercontrols logical channels, transport channels, and physical channels inrelation to the configuration, reconfiguration, and release of RBs. AnRB is a logical path provided by the L1 and L2 for data delivery betweenthe UE and the network. That is, the RB signifies a service provided theL2 for data transmission between the UE and E-UTRAN. The configurationof the RB implies a process for specifying a radio protocol layer andchannel properties to provide a particular service and for determiningrespective detailed parameters and operations. The RB is classified intotwo types, i.e., a signaling RB (SRB) and a data RB (DRB). The SRB isused as a path for transmitting an RRC message in the control plane. TheDRB is used as a path for transmitting user data in the user plane.

Referring to FIG. 3-(a), the RLC and MAC layers (terminated in the eNBon the network side) may perform functions such as scheduling, automaticrepeat request (ARQ), and hybrid automatic repeat request (HARQ). ThePDCP layer (terminated in the eNB on the network side) may perform theuser plane functions such as header compression, integrity protection,and ciphering.

Referring to FIG. 3-(b), the RLC and MAC layers (terminated in the eNBon the network side) may perform the same functions for the controlplane. The RRC layer (terminated in the eNB on the network side) mayperform functions such as broadcasting, paging, RRC connectionmanagement, RB control, mobility functions, and UE measurement reportingand controlling. The NAS control protocol (terminated in the MME ofgateway on the network side) may perform functions such as a SAE bearermanagement, authentication, LTE_IDLE mobility handling, pagingorigination in LTE_IDLE, and security control for the signaling betweenthe gateway and UE.

An RRC state indicates whether an RRC layer of the UE is logicallyconnected to an RRC layer of the E-UTRAN. The RRC state may be dividedinto two different states such as an RRC connected state and an RRC idlestate. When an RRC connection is established between the RRC layer ofthe UE and the RRC layer of the E-UTRAN, the UE is in RRC_CONNECTED, andotherwise the UE is in RRC_IDLE. Since the UE in RRC_CONNECTED has theRRC connection established with the E-UTRAN, the E-UTRAN may recognizethe existence of the UE in RRC_CONNECTED and may effectively control theUE. Meanwhile, the UE in RRC_IDLE may not be recognized by the E-UTRAN,and a CN manages the UE in unit of a TA which is a larger area than acell. That is, only the existence of the UE in RRC_IDLE is recognized inunit of a large area, and the UE must transition to RRC_CONNECTED toreceive a typical mobile communication service such as voice or datacommunication.

In RRC_IDLE state, the UE may receive broadcasts of system informationand paging information while the UE specifies a discontinuous reception(DRX) configured by NAS, and the UE has been allocated an identification(ID) which uniquely identifies the UE in a tracking area and may performpublic land mobile network (PLMN) selection and cell re-selection. Also,in RRC_IDLE state, no RRC context is stored in the eNB.

In RRC_CONNECTED state, the UE has an E-UTRAN RRC connection and acontext in the E-UTRAN, such that transmitting and/or receiving datato/from the eNB becomes possible. Also, the UE can report channelquality information and feedback information to the eNB. InRRC_CONNECTED state, the E-UTRAN knows the cell to which the UE belongs.Therefore, the network can transmit and/or receive data to/from UE, thenetwork can control mobility (handover and inter-radio accesstechnologies (RAT) cell change order to GSM EDGE radio access network(GERAN) with network assisted cell change (NACC)) of the UE, and thenetwork can perform cell measurements for a neighboring cell.

In RRC_IDLE state, the UE specifies the paging DRX cycle. Specifically,the UE monitors a paging signal at a specific paging occasion of everyUE specific paging DRX cycle. The paging occasion is a time intervalduring which a paging signal is transmitted. The UE has its own pagingoccasion.

A paging message is transmitted over all cells belonging to the sametracking area. If the UE moves from one TA to another TA, the UE willsend a tracking area update (TAU) message to the network to update itslocation.

When the user initially powers on the UE, the UE first searches for aproper cell and then remains in RRC_IDLE in the cell. When there is aneed to establish an RRC connection, the UE which remains in RRC_IDLEestablishes the RRC connection with the RRC of the E-UTRAN through anRRC connection procedure and then may transition to RRC_CONNECTED. TheUE which remains in RRC_IDLE may need to establish the RRC connectionwith the E-UTRAN when uplink data transmission is necessary due to auser's call attempt or the like or when there is a need to transmit aresponse message upon receiving a paging message from the E-UTRAN.

It is known that different cause values may be mapped o the signaturesequence used to transmit messages between a UE and eNB and that eitherchannel quality indicator (CQI) or path loss and cause or message sizeare candidates for inclusion in the initial preamble.

When a UE wishes to access the network and determines a message to betransmitted, the message may be linked to a purpose and a cause valuemay be determined. The size of the ideal message may be also bedetermined by identifying all optional information and differentalternative sizes, such as by removing optional information, or analternative scheduling request message may be used.

The UE acquires necessary information for the transmission of thepreamble, UL interference, pilot transmit power and requiredsignal-to-noise ratio (SNR) for the preamble detection at the receiveror combinations thereof. This information must allow the calculation ofthe initial transmit power of the preamble. It is beneficial to transmitthe UL message in the vicinity of the preamble from a frequency point ofview in order to ensure that the same channel is used for thetransmission of the message.

The UE should take into account the UL interference and the UL path lossin order to ensure that the network receives the preamble with a minimumSNR. The UL interference can be determined only in the eNB, andtherefore, must be broadcast by the eNB and received by the UE prior tothe transmission of the preamble. The UL path loss can be considered tobe similar to the DL path loss and can be estimated by the UE from thereceived RX signal strength when the transmit power of some pilotsequence of the cell is known to the UE.

The required UL SNR for the detection of the preamble would typicallydepend on the eNB configuration, such as a number of Rx antennas andreceiver performance. There may be advantages to transmit the ratherstatic transmit power of the pilot and the necessary UL SNR separatelyfrom the varying UL interference and possibly the power offset requiredbetween the preamble and the message.

The initial transmission power of the preamble can be roughly calculatedaccording to the following formula:

Transmit power=TransmitPilot−RxPilot+ULInterference+Offset+SNRRequired

Therefore, any combination of SNRRequired, ULInterference, TransmitPilotand Offset can be broadcast. In principle, only one value must bebroadcast. This is essentially in current UMTS systems, although the ULinterference in 3GPP LTE will mainly be neighboring cell interferencethat is probably more constant than in UMTS system.

The UE determines the initial UL transit power for the transmission ofthe preamble as explained above. The receiver in the eNB is able toestimate the absolute received power as well as the relative receivedpower compared to the interference in the cell. The eNB will consider apreamble detected if the received signal power compared to theinterference is above an eNB known threshold.

The UE performs power ramping in order to ensure that a UE can bedetected even if the initially estimated transmission power of thepreamble is not adequate. Another preamble will most likely betransmitted if no ACK or NACK is received by the UE before the nextrandom access attempt. The transmit power of the preamble can beincreased, and/or the preamble can be transmitted on a different ULfrequency in order to increase the probability of detection. Therefore,the actual transmit power of the preamble that will be detected does notnecessarily correspond to the initial transmit power of the preamble asinitially calculated by the UE.

The UE must determine the possible UL transport format. The transportformat, which may include MCS and a number of resource blocks thatshould be used by the UE, depends mainly on two parameters, specificallythe SNR at the eNB and the required size of the message to betransmitted.

In practice, a maximum UE message size, or payload, and a requiredminimum SNR correspond to each transport format. In UMTS, the UEdetermines before the transmission of the preamble whether a transportformat can be chosen for the transmission according to the estimatedinitial preamble transmit power, the required offset between preambleand the transport block, the maximum allowed or available UE transmitpower, a fixed offset and additional margin. The preamble in UMTS neednot contain any information regarding the transport format selected bythe EU since the network does not need to reserve time and frequencyresources and, therefore, the transport format is indicated togetherwith the transmitted message.

The eNB must be aware of the size of the message that the UE intends totransmit and the SNR achievable by the UE in order to select the correcttransport format upon reception of the preamble and then reserve thenecessary time and frequency resources. Therefore, the eNB cannotestimate the SNR achievable by the EU according to the received preamblebecause the UE transmit power compared to the maximum allowed orpossible UE transmit power is not known to the eNB, given that the UEwill most likely consider the measured path loss in the DL or someequivalent measure for the determination of the initial preambletransmission power.

The eNB could calculate a difference between the path loss estimated inthe DL compared and the path loss of the UL. However, this calculationis not possible if power ramping is used and the UE transmit power forthe preamble does not correspond to the initially calculated UE transmitpower. Furthermore, the precision of the actual UE transmit power andthe transmit power at which the UE is intended to transmit is very low.Therefore, it has been proposed to code the path loss or CQI estimationof the downlink and the message size or the cause value in the UL in thesignature.

Proximity Services (ProSe) are described. It may be refer to 3GPP TR23.703 V0.4.1 (2013-06). The ProSe may be a concept including adevice-to-device (D2D) communication. Hereinafter, the ProSe may be usedby being mixed with a device-to-device (D2D).

ProSe direct communication means a communication between two or more UEsin proximity that are ProSe-enabled, by means of user plane transmissionusing E-UTRA technology via a path not traversing any network node.ProSe-enabled UE means a UE that supports ProSe requirements andassociated procedures. Unless explicitly stated otherwise, aProSe-enabled UE refers both to a non-public safety UE and a publicsafety UE. ProSe-enabled public safety UE means a ProSe-enabled UE thatalso supports ProSe procedures and capabilities specific to publicsafety. ProSe-enabled non-public safety UE means a UE that supportsProSe procedures and but not capabilities specific to public safety.ProSe direct discovery means a procedure employed by a ProSe-enabled UEto discover other ProSe-enabled UEs in its vicinity by using only thecapabilities of the two UEs with 3GPP LTE rel-12 E-UTRA technology.EPC-level ProSe discovery means a process by which the EPC determinesthe proximity of two ProSe-enabled UEs and informs them of theirproximity.

When the registered public land mobile network (PLMN), ProSe directcommunication path and coverage status (in coverage or out of coverage)are considered, there are a number of different possible scenarios.Different combinations of direct data paths and in-coverage andout-of-coverage may be considered.

FIG. 5 and FIG. 6 show ProSe direct communication scenarios without arelay. FIG. 5-(a) shows a case that UE1 and UE2 are out of coverage.FIG. 5-(b) shows a case that UE1 is in coverage and in PLMN A, and UE2is out of coverage. FIG. 5-(c) shows a case that UE1 and UE2 are incoverage and in PLMN A, and UE1 and UE2 shares the same PLMN A and thesame cell. FIG. 5-(d) shows a case that UE1 and UE2 are in coverage andin the same PLMN A, but UE1 and UE2 are in different cells each other.FIG. 6-(a) shows a case that UE1 and UE2 are in coverage, but UE1 andUE2 are in different PLMNs (i.e., PLMN A/B) and different cells eachother. UE1 and UE2 are in both cells' coverage. FIG. 6-(b) shows a casethat UE1 and UE2 are in coverage, but UE1 and UE2 are in different PLMNs(i.e., PLMN A/B) and different cells each other. UE1 is in both cells'coverage and UE2 is in serving cell's coverage. FIG. 6-(c) shows a casethat UE1 and UE2 are in coverage, but UE1 and UE2 are in different PLMNs(i.e., PLMN A/B) and different cells each other. UE1 and UE2 are in itsown serving cell's coverage. In the description above, “in coverage andin PLMN A” means that the UE is camping on the cell of the PLMN A andunder the control of the PLMN A.

Two different modes for ProSe direct communication one-to-one may besupported.

-   -   Network independent direct communication: This mode of operation        for ProSe direct communication does not require any network        assistance to authorize the connection and communication is        performed by using only functionality and information local to        the UE. This mode is applicable only to pre-authorized        ProSe-enabled public safety UEs, regardless of whether the UEs        are served by E-UTRAN or not.    -   Network authorized direct communication: This mode of operation        for ProSe direct communication always requires network        assistance and may also be applicable when only one UE is        “served by E-UTRAN” for public safety UEs. For non-public safety        UEs both UEs must be “served by E-UTRAN”.

FIG. 7 shows reference architecture for ProSe. Referring to FIG. 7, thereference architecture for ProSe includes E-UTRAN, EPC, a plurality ofUEs having ProSe applications, ProSe application server, and ProSefunction. The EPC represents the E-UTRAN core network architecture. TheEPC may include entities such as MME, S-GW, P-GW, policy and chargingrules function (PCRF), home subscriber server (HSS), etc. The ProSeapplication servers are users of the ProSe capability for building theapplication functionality. In the public safety cases, they may bespecific agencies (PSAP), or in the commercial cases social media. Theseapplications may be defined outside the 3GPP architecture but there maybe reference points towards 3GPP entities. The application server cancommunicate towards an application in the UE. Applications in the UE usethe ProSe capability for building the application functionality. Examplemay be for communication between members of public safety groups or forsocial media application that requests to find buddies in proximity.

The ProSe function in the network (as part of EPS) defined by 3GPP has areference point towards the ProSe application server, towards the EPCand the UE. The functionality may include at least one of followings.But the functionality may not be restricted to the followings.

-   -   Interworking via a reference point towards the 3rd party        applications    -   Authorization and configuration of the UE for discovery and        direct communication    -   Enable the functionality of the EPC level ProSe discovery    -   ProSe related new subscriber data and handling of data storage,        and also handling of ProSe identities    -   Security related functionality    -   Provide control towards the EPC for policy related functionality    -   Provide functionality for charging (via or outside of EPC, e.g.,        offline charging)

Reference points/interfaces in the reference architecture for ProSe aredescribed.

-   -   PC1: It is the reference point between the ProSe application in        the UE and in the ProSe application server. It is used to define        application level signaling requirements.    -   PC2: It is the reference point between the ProSe application        server and the ProSe function. It is used to define the        interaction between ProSe application server and ProSe        functionality provided by the 3GPP EPS via ProSe function. One        example may be for application data updates for a ProSe database        in the ProSe function. Another example may be data for use by        ProSe application server in interworking between 3GPP        functionality and application data, e.g., name translation.    -   PC3: It is the reference point between the UE and ProSe        function. It is used to define the interaction between UE and        ProSe function. An example may be to use for configuration for        ProSe discovery and communication.    -   PC4: It is the reference point between the EPC and ProSe        function. It is used to define the interaction between EPC and        ProSe function. Possible use cases may be when setting up a        one-to-one communication path between UEs or when validating        ProSe services (authorization) for session management or        mobility management in real time.    -   PC5: It is the reference point between UE to UE used for control        and user plane for discovery and communication, for relay and        one-to-one communication (between UEs directly and between UEs        over LTE-Uu).    -   PC6: This reference point may be used for functions such as        ProSe discovery between users subscribed to different PLMNs.    -   SGi: In addition to the relevant functions via SGi, it may be        used for application data and application level control        information exchange.

ProSe direct communication is a mode of communication whereby two publicsafety UEs can communicate with each other directly over the PC5interface. This communication mode is supported when the UE is served byE-UTRAN and when the UE is outside of E-UTRA coverage.

The ProSe-enabled UE may operate in two modes for resource allocation.In mode 1, resource allocation is scheduled by the eNB. In mode 1, theUE may need to be RRC_CONNECTED in order to transmit data. The UE mayrequest transmission resources from the eNB. The eNB may scheduletransmission resources for transmission of scheduling assignment(s) anddata. The UE may send a scheduling request (dedicated scheduling request(D-SR) or random access) to the eNB followed by a ProSe buffer statusreport (BSR). Based on the BSR, the eNB may determine that the UE hasdata for a ProSe direct communication transmission and estimate theresources needed for transmission. In mode, 2, a UE on its own selectsresources autonomously from resource pools to transmit schedulingassignment and data. If the UE is out of coverage, the UE may only usemode 2. If the UE is in coverage, the UE may use mode 1 or mode 2according to configuration of the eNB. When there are no exceptionalconditions, the UE may change from mode 1 to mode 2 or mode 2 to mode 1only if it is configured by the eNB. If the UE is in coverage, the UEshall use only the mode indicated by eNB configuration unless one of theexceptional cases occurs.

ProSe direct discovery is defined as the procedure used by theProSe-enabled UE to discover other ProSe-enabled UE(s) in its proximityusing E-UTRA direct radio signals via the PC5 interface. ProSe directdiscovery is supported only when the UE is served by E-UTRAN.

There are two types of resource allocation for discovery informationannouncement. Type 1 is a resource allocation procedure where resourcesfor announcing of discovery information are allocated on a non UEspecific basis. The eNB may provide the UE(s) with the resource poolconfiguration used for announcing of discovery information. Theconfiguration may be signaled in system information block (SIB). The UEautonomously selects radio resource(s) from the indicated resource pooland announce discovery information. The UE may announce discoveryinformation on a randomly selected discovery resource during eachdiscovery period. Type 2 is a resource allocation procedure whereresources for announcing of discovery information are allocated on a perUE specific basis. The UE in RRC_CONNECTED may request resource(s) forannouncing of discovery information from the eNB via RRC. The eNB mayassign resource(s) via RRC. The resources may be allocated within theresource pool that is configured in UEs for monitoring.

FIG. 8 shows an example of one-step ProSe direct discovery procedure. InFIG. 8, two UEs are running the same ProSe-enabled application and it isassumed that the users of those UEs have a “friend” relationship on theconsidered application. The “3GPP Layers” shown in FIG. 8 correspond tothe functionality specified by 3GPP that enables mobile applications inthe UE to use ProSe discovery services.

UE-A and UE-B run a ProSe-enabled application, which discovers andconnects with an associated application server in the network. As anexample, this application could be a social networking application. Theapplication server could be operated by the 3GPP network operator or bya third-party service provider. When operated by a third-party provider,a service agreement is required between the third-party provider and the3GPP operator in order to enable communication between the ProSe Serverin the 3GPP network and the application server.

1. Regular application-layer communication takes place between themobile application in UE-A and the application server in the network.

2. The ProSe-enabled application in UE-A retrieves a list ofapplication-layer identifiers, called “friends”. Typically, suchidentifiers have the form of a network access identifier.

3. The ProSe-enabled application wants to be notified when one of UE-A'sfriends is in the vicinity of UE-A. For this purpose, it requests fromthe 3GPP layers to retrieve private expressions codes (i) for the userof UE-A (with an application-layer identity) and (ii) for each one ofhis friends.

4. The 3GPP layers delegate the request to a ProSe server in the 3GPPnetwork. This server can be located either in home PLMN (HPLMN) or in avisited PLMN (VPLMN). Any ProSe server that supports the consideredapplication can be used. The communication between the UE and ProSeserver can take place either over the IP layer or below the IP layer. Ifthe application or the UE is not authorized to use ProSe discovery, thenthe ProSe server rejects the request.

5. The ProSe server maps all provided application-layer identities toprivate expression codes. For example, the application-layer identity ismapped to the private expression code. This mapping is based onparameters retrieved from the application server in the network (e.g.,mapping algorithm, keys, etc.) thus the derived private expression codecan be globally unique. In other words, any ProSe server requested toderive the private expression of the application-layer identity for aspecific application, it will derive the same private expression code.The mapping parameters retrieved from the application server describehow the mapping should be done. In this step, the ProSe server and/orthe application server in the network authorize also the request toretrieve expression codes for a certain application and from a certainuser. It is ensured, for example, that a user can retrieves expressioncodes only for his friends.

6. The derived expression codes for all requested identities are sent tothe 3GPP layers, where they are stored for further use. In addition, the3GPP layers notify the ProSe-enabled application that expression codesfor the requested identities and application have been successfullyretrieved. However, the retrieved expression codes are not sent to theProSe-enabled application.

7. The ProSe-enabled application requests from the 3GPP layers to startdiscovery, i.e., attempt to discover when one of the provided “friends”is in the vicinity of UE-A and, thus, direct communication is feasible.As a response, UE-A announces the expression code of theapplication-layer identity for the considered application. The mappingof this expression code to the corresponding application-layer identifycan only be performed by the friends of UE-A, who have also received theexpression codes for the considered application.

8. UE-B also runs the same ProSe-enabled application and has executedsteps 3-6 to retrieve the expression codes for friends. In addition, the3GPP layers in UE-B carry out ProSe discovery after being requested bythe ProSe-enabled application.

9. When UE-B receives the ProSe announcement from UE-A, it determinesthat the announced expression code is known and maps to a certainapplication and to the application-layer identity. The UE-B candetermine the application and the application identity that correspondsto the received expression code because it has also received theexpression code for the application-layer identity (UE-A is included inthe friend list of UE-B).

The steps 1-6 in the above procedure can only be executed when the UE isinside the network coverage. However, these steps are not requiredfrequently. They are only required when the UE wants to update or modifythe friends that should be discovered with ProSe direct discovery. Afterreceiving the requested expression codes from the network, the ProSediscovery (steps 7 and 9) can be conducted either inside or outside thenetwork coverage.

It is noted that an expression code maps to a certain application and toa certain application identity. Thus when a user runs the sameProSe-enabled application on multiple UEs, each UE announces the sameexpression code.

FIG. 9 shows an example of two-steps ProSe direct discovery procedure.

1. The user of UE1 (the discoverer) wishes to discover whether there areany members of a specific group communication service enabler (GCSE)group in proximity. UE1 broadcasts a targeted discovery request messagecontaining the unique App group ID (or the Layer-2 group ID) of thetargeted GCSE group. The targeted discovery request message may alsoinclude the discoverer's unique identifier (App personal ID of user 1).The targeted discovery request message is received by UE2, UE3, UE4 andUE5. Apart from the user of UE5, all other users are members of therequested GCSE group and their UEs are configured accordingly.

2a-2c. Each one of UE2, UE3 and UE4 responds directly to UE1 with atargeted discovery response message which may contain the unique Apppersonal ID of its user. In contrast, UE5 sends no response message.

In three step procedure, UE1 may respond to the targeted discoveryresponse message by sending a discovery confirm message.

Hereinafter, a method for handling radio resources for D2D operation(hereinafter, D2D resources) upon mobility according to an embodiment ofthe present invention is described. During the handover preparationprocedure, if D2D resources have been configured to the UE by the sourcecell, information the D2D resources should be forwarded to the targetcell. The information on D2D resources may further include informationon D2D resources of neighbor cell(s) in perspective of the serving cell,if available. Then, upon receiving the information on D2D resources, thetarget cell may know that the UE has been using or being interested in aD2D operation and thus the target cell may properly configure the UE tosupport continuity of the D2D operation after handover is completed.While the handover procedure, the target cell may transmit various kindsof information on D2D resources to the UE.

FIG. 10 shows an example of a method for performing a D2D operationaccording to an embodiment of the present invention. In step S100, theUE receives information on radio resources allowed for a D2D operationin a target cell during a handover procedure. In step S110, the UEperforms the D2D operation in the target cell based on receivedinformation on radio resources.

The information on radio resources allowed for a D2D operation in thetarget cell may be included in a handover command message. The D2D radioresources may indicate radio resources that are authorized by thenetwork for D2D transmission. Here, D2D transmission may be transmissionof IP packet for D2D communication or transmission of MAC protocol dataunit (PDU) for D2D discovery signal/message or transmission of IP packetfor D2D discovery signal/message. The D2D radio resources may includetime period or time period pattern in which D2D communication can takeplace. The D2D radio resources may include time slot or time slotpattern in which D2D communication can take place. The D2D radioresources may include subframe or subframe pattern in which D2Dcommunication can take place. The D2D radio resources may include asubframe in which D2D communication can take place.

Upon receiving the information on the D2D radio resources allowed in thetarget cell, the UE performs the D2D operation in the target cell basedon received information on the D2D radio resources in the target cell.That is, the UE may restrict its radio resources for the D2D operationin the target cell by considering the received information on the D2Dradio resources allowed in the target cell. The information on the D2Dradio resources may include information on D2D radio resources ofneighbor cell (in perspective of the target cell). The UE may alsoconsider the D2D radio resources of the source cell to determine theactual radio resources to be allowed for D2D operation in the targetcell.

Meanwhile, it is possible that upon performing a handover procedure orconnection re-establishment procedure, the D2D resources may beconsidered as invalid (i.e., unusable). In this case, the UE shouldsuspend using D2D resources for a D2D operation.

Alternatively, it is possible that upon performing the handoverprocedure or connection re-establishment procedure, the UE maytemporarily consider that the D2D resources, which were configuredbefore the handover procedure or connection re-establishment procedure,to be valid even if the UE changes its serving cell due to the handoverprocedure or connection re-establishment procedure. Then, the UE maycontinue to perform the D2D operation during the handover procedure orconnection re-establishment procedure, and therefore, minimize theinterruption of the D2D operation. If the concerned procedure (e.g.,handover procedure) ends, the UE may stop using the D2D resourcesconfigured before the concerned procedure and start to use the D2Dresources configured during the concerned procedure.

Alternatively, it is possible that upon performing the handoverprocedure or connection re-establishment procedure, the UE may start toperform a D2D operation by using the D2D resources configured during theconcerned procedure (e.g., handover procedure), in order to minimize theinterruption of the D2D operation.

FIG. 11 shows an example of a method for performing a D2D operationaccording to another embodiment of the present invention. In step S200,the UE performs a handover procedure or a connection re-establishmentprocedure. In step S210, the UE suspends using D2D resources. When theUE suspends using D2D resources, the UE may suspend both D2Dtransmission using D2D resources and reception using D2D resources. Or,the UE may only suspend D2D transmission using D2D resources butcontinues reception using D2D resources.

The suspended D2D resources may be considered as valid again. If the UEperforms the handover procedure, the suspended D2D resources may beconsidered as valid again after the first connection reconfigurationprocedure is performed after the handover procedure is completed.Alternatively, the suspended D2D resources may be considered as validagain after the UE receives an explicit indication from the network uponor after the handover procedure. The explicit indication may allow theUE to activate the suspended D2D resources. The explicit indication maybe included in the reconfiguration message.

If the UE performs the connection re-establishment procedure, thesuspended D2D resources may be considered as valid after the connectionre-establishment procedure is completed. Alternatively, the suspendedD2D resources may be considered as valid after the first connectionreconfiguration procedure is performed after the connectionre-establishment procedure is completed. Alternatively, the suspendedD2D resources may be considered as valid again after the UE receives anexplicit indication from the network upon or after the connectionre-establishment procedure. The explicit indication may allow the UE toactivate the suspended D2D resources. The explicit indication may beincluded in the RRC connection reestablishment message or RRC connectionreconfiguration message. The explicit indication may be an explicitgrant which is transmitted by the network to grant D2D transmission ofthe UE.

Back to FIG. 11, in step S220, the UE resumes suspended D2D resources.If the suspended D2D resources are considered as valid again, the UE mayresume the suspended D2D resources, and accordingly, resume thesuspended D2D operation.

When the D2D resources are suspended or resumed, the UE may indicatethis to the upper layer that is responsible for D2D service. The upperlayer may be a layer which is in charge of scheduling or resourceallocation, or in charge of overall UE proximity service, or applicationlayer that can manage overall D2D service.

During the handover procedure or connection re-establishment procedure,the UE may stop performing D2D operations related to a normal D2Dservice while the UE keeps all or part of D2D operations related to ahigh priority D2D service. For example, a commercial D2D service, e.g.,D2D discovery for the commercial, may be considered as a normal D2Dservice, while public safety communication, e.g., D2D communication formission critical push to talk (MCPTT), may be considered as highpriority D2D service.

FIG. 12 is a block diagram showing wireless communication system toimplement an embodiment of the present invention.

An entity of the network 800 may include a processor 810, a memory 820and a radio frequency (RF) unit 830. The processor 810 may be configuredto implement proposed functions, procedures and/or methods described inthis description. Layers of the radio interface protocol may beimplemented in the processor 810. The memory 820 is operatively coupledwith the processor 810 and stores a variety of information to operatethe processor 810. The RF unit 830 is operatively coupled with theprocessor 810, and transmits and/or receives a radio signal.

A UE 900 may include a processor 910, a memory 920 and a RF unit 930.The processor 910 may be configured to implement proposed functions,procedures and/or methods described in this description. Layers of theradio interface protocol may be implemented in the processor 910. Thememory 920 is operatively coupled with the processor 910 and stores avariety of information to operate the processor 910. The RF unit 930 isoperatively coupled with the processor 910, and transmits and/orreceives a radio signal.

The processors 810, 910 may include application-specific integratedcircuit (ASIC), other chipset, logic circuit and/or data processingdevice. The memories 820, 920 may include read-only memory (ROM), randomaccess memory (RAM), flash memory, memory card, storage medium and/orother storage device. The RF units 830, 930 may include basebandcircuitry to process radio frequency signals. When the embodiments areimplemented in software, the techniques described herein can beimplemented with modules (e.g., procedures, functions, and so on) thatperform the functions described herein. The modules can be stored inmemories 820, 920 and executed by processors 810, 910. The memories 820,920 can be implemented within the processors 810, 910 or external to theprocessors 810, 910 in which case those can be communicatively coupledto the processors 810, 910 via various means as is known in the art.

In view of the exemplary systems described herein, methodologies thatmay be implemented in accordance with the disclosed subject matter havebeen described with reference to several flow diagrams. While forpurposed of simplicity, the methodologies are shown and described as aseries of steps or blocks, it is to be understood and appreciated thatthe claimed subject matter is not limited by the order of the steps orblocks, as some steps may occur in different orders or concurrently withother steps from what is depicted and described herein. Moreover, oneskilled in the art would understand that the steps illustrated in theflow diagram are not exclusive and other steps may be included or one ormore of the steps in the example flow diagram may be deleted withoutaffecting the scope and spirit of the present disclosure.

1. A method for performing, by a user equipment (UE), a device-to-device(D2D) operation in a wireless communication system, the methodcomprising: receiving information on radio resources allowed for a D2Doperation in a target cell during a handover procedure; and performingthe D2D operation in the target cell based on received information onradio resources.
 2. The method of claim 1, wherein the information onradio resources is received via a handover command message from thetarget cell.
 3. The method of claim 1, wherein the information on radioresources indicate radio resources authorized by a network for D2Dtransmission.
 4. The method of claim 3, wherein the D2D transmissionincludes at least one of transmission of Internet protocol (IP) packetfor D2D communication or D2D discovery signal, or transmission of mediaaccess control (MAC) protocol data unit (PDU) for D2D discovery signal.5. The method of claim 1, wherein the information on radio resourcesindicate a time period or a pattern of the time period in which the D2Doperation is able to be performed.
 6. The method of claim 5, wherein thetime period is one of a time slot or a subframe.
 7. The method of claim1, wherein the information on radio resources includes information onradio resources for the D2D operation of a neighbor cell of the targetcell.
 8. A user equipment (UE) in a wireless communication system, theUE comprising: a radio frequency (RF) unit for transmitting or receivinga radio signal; and a processor coupled to the RF unit, and configuredto: receive information on radio resources allowed for adevice-to-device (D2D) operation in a target cell during a handoverprocedure; and perform the D2D operation in the target cell based onreceived information on radio resources.
 9. A method for performing, bya user equipment (UE), device-to-device (D2D) operation in a wirelesscommunication system, the method comprising: performing a handoverprocedure or a connection re-establishment procedure; suspending usingD2D resources; and resuming suspended D2D resources.
 10. The method ofclaim 9, wherein the D2D resources are suspended for both D2Dtransmission and D2D reception.
 11. The method of claim 9, wherein theD2D resources are suspended only for D2D transmission.
 12. The method ofclaim 9, wherein the suspended D2D resources are resumed after a firstreconfiguration procedure is performed after the handover procedure orthe connection re-establishment procedure is completed.
 13. The methodof claim 9, wherein the suspended D2D resources are resumed afterreceiving an indication which indicates resuming the suspended D2Dresources from a network.
 14. The method of claim 13, wherein theindication is received via a reconfiguration message.
 15. The method ofclaim 9, wherein the suspended D2D resources are resumed after theconnection re-establishment procedure is completed.